I’ve been interested in sundials for ages. Tracking the sun’s path by observing the shadow of a stick is an ancient form of astronomy, and a gateway into geometry (literally “measuring the earth”).
A sundial marks the solstices and equinoxes, and enables measurement of the cardinal directions of north, south, east, and west: at the peak of its daily journey through the sky, the sun throws a north-south shadow; on the equinoxes, the shadow of the sun draws an east-west line. Ancient sundials acted as calendars, showing precisely when the sun returned to a given spot in its annual journey from south to north and back again.
Until recently, my interest in sundials has been abstract: I hadn’t actually built a sundial, because they require a lot of precise work. Now that I have a 3D printer, I’ve realized that building a sundial is easy!
The simplest type of sundial to lay out is an equatorial dial: a disk divided into 24 equal parts, facing north. For various reasons I prefer a type that is difficult to make by hand, but very easy to create with a 3D printer: a hemispherium, or Scaphe. A Scaphe is basically a portion of a sphere with a marker in the center and hour lines marked on the sphere.
For my Scaphe, I cut the sphere back to only the part that the sun’s shadow will fall on: the space between the tropics, and only points below the horizon. It was easy to design a 3D sphere, add curved hour lines to its interior, add a ring for the equinox (the celestial equator), cut it on the tropics of Cancer and Capricorn, tilt it for the latitude at my home, then cut the horizon. The result is the image at the top of this blog.
In the picture, the shadow of the “sun” (a nearby lamp) is showing about 9:30am Standard Time (10:30am Daylight Time), in late April or July. The horizontal rotational position of the shadow gives the hour; the vertical position of the shadow gives the date (actually, one of two dates, because in the second half of the year the sun retraces the path it made during the first half).
Next I plan to add more “Furniture” (sundial markers): the Standard and Daylight Saving Time hour numbers and perhaps a labels for the equator and tropics.
I’ll write more about this sundial as I develop and test it.